                (a) Technical Field        
The present invention relates to a high heat radiation composite including a hybrid filler comprising expanded graphite filled with expandable polymeric beads, which is dispersed in a matrix polymer, and a fabrication method thereof. More particularly, it relates to a high heat radiation composite including a hybrid filler comprising expanded graphite filled with expandable polymeric beads that has been heat-treated and dispersed into a matrix polymer by an extrusion/injection process. The composite has better heat radiation characteristics than a typical heat-radiating composite, overcomes thermal anisotropy, and may be utilized as a material for battery cases and housing and plates interposed between pouch cells in a cell module of a battery system for an electric vehicle.                (b) Background Art        
Generally, thermal runaway is a phenomenon that hinders the efficiency and reliability of batteries due to local temperature differences or high heat caused by high-speed charging, high power, and repeated charging occurs in batteries for electric vehicles. Thermal runaway may result from a deficiency or lack of external thermal diffusion capacity compared to the internal heat generated by batteries.
Generally, materials that are being used for battery cases and housings have a mineral filler, e.g., an incombustible filler is filled in a plastic base material such as PC+ABS, PA and PP by 20 wt % to 30 wt %. These materials provide beneficial characteristics such as flame resistance, chemical resistance, insulation, and durability. However, these materials do not provide good heat radiation characteristics. In the case of heat radiation composites that are being developed, thermal anisotropy generally occurs due to the orientation of a filler in an injection process; however, there is a significant limitation in achieving high heat conduction in such composites because a heat transfer resistance occurs at the interfaces between components such as, for example, filler and resin.
Generally, high heat radiation fillers are used as a polymer-based heat radiation composite material, and high heat radiation fillers having a planar shape are advantageous. In the case of fibrous or globular fillers, the contact between fillers is not a plane contact but rather a point contact, and thus the transfer efficiency of the lattice vibrator (phonon) may be rapidly reduced. Examples of planar high heat radiation filler include boron nitride and graphite.
When a sample is manufactured by injection-molding a composite resin densely filled with planar particles, the planar particles are oriented in one direction by a shear force applied in the injection direction, causing anisotropy of thermal conductivity. In addition, a densely filled heat radiation composite material manufactured by typical injection molding has limitations of workability reduction due to low resin flowability, high price of filler, and weight increase.
Hybrid fillers in which two types of filler, planar fillers and globular fillers, have a limitation in that their heat transfer efficiency cannot be maximized due to weight increase by the thick filling of the whole filler and relatively low thermal conductivity of globular particles. Also, the thermal energy transfer capacity is reduced due to scattering of phonon in pores of the filled particles and the polymer resin filled in between the globular particles and planar particles.
Typical examples of heat transfer resistance factors of polymer-based heat radiation composite materials include an interfacial resistance between matrix resin and filler, a resistance due to defects in filler, and a resistance occurring in a contact portion between fillers. These resistance factors cause the heat conduction efficiency to be significantly reduced.
The interfacial resistance between matrix resin and filler is in association with the interfacial stability. For this, resin needs to be fully impregnated into the surface of filler, and thus the mechanical properties can simultaneously increase. The resistance due to defects in filler is determined by physical factors in the selection stage and the pretreatment stage of filler. The interfacial resistance at the contact points between fillers may be minimized by maximizing a surface contact between fillers. For this, a densification process for the surface contact between planar particles is needed. Since the densification process of planar particles is essential for the efficiency of phonon transfer, but may cause a collapse of a bulky network, the densification process of planar particles needs to be induced while maintaining the network of particles.
Attempts to overcome these problems have implemented a composite containing expanded graphite or expandable polymer. For example, a polymer/graphene nano composite material with good conductivity manufactured by effectively dispersing graphite-based graphite materials such as graphene, expanded graphite, or undenatured graphite in a polymer matrix. Unfortunately, since graphite has a small amount of polar group on the surface thereof, it is difficult to effectively disperse graphite in a polymer. Also, since planar particles are not densified, the thermal conductivity is low.
Other attempts to overcome these problems have implemented a nano composite with an expanded graphite/epoxy nano composite composition and a nano composite, which has excellent thermal and mechanical characteristics. For example, a nano composite, that is manufactured by fusing and mixing acid-treated and heat-treated expanded graphite with epoxy resin, may have limited anisotropy due to the particle shape of the expanded graphite.
Accordingly, there is a need for a material that can effectively radiate heat generated in batteries to increase the lifespan and the reliability of a high capacity battery package for an electric vehicle, which can be disposed between a pouch type of lithium ion batteries as an interfacial plate material or can be used for upper and lower plate covers (hereinafter, referred to as housing) for effectively coupling a fixed battery cell and an interfacial plate module and increasing their durability.